DEFINITION: A pager is a small telecommunications device that receives (and, in some cases, transmits) alert signals and/or short messages. This type of device is convenient for people expecting telephone calls, but who are not near a telephone set to make or return calls immediately.

A typical one-way pager fits easily in a shirt pocket; some are as small as a wristwatch. A miniature, short-range wireless  … 
Definition continues below.

PAGERS DEFINITION (continued): …  receiver captures a message, usually accompanied by a beep. (This is why the device is also known as a beeper). The simplest one-way pagers display the return-call telephone number of the person who sent the message. Alternatively, a code can be displayed that indicates which of several designated parties is requesting a return phone call. Sophisticated one-way pagers can display short text messages.

Until recently, pagers were designed as receive-only devices. There are at least two reasons for this. First, if two-way communication is needed, cell phones are available for that purpose. Second, … 
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